Let me preface this review by saying I have no idea how one goes about writing a song, but I've always had an interest in music, audio recording, and midi technologies. I took piano lessons as a child just long enough to memorize about a half dozen songs that I play in constant rotation to my family's distress. Other than that, I have no music abilities.
Now with all that said, I love GarageBand for the iPad. Apple developed an iPad version of their popular desktop application and have essentially created an ultra-portable recording studio. The app comes with built-in "instruments" that you can use with the iPad's touchscreen to lay down multiple tracks. The obvious ones are a touchscreen piano keyboard and drum kit which have great sound samples.
The instrument that makes GarageBand a killer app is the "Smart Guitar" instrument. The screen shows several chords. You can tap on the chord letter to play the entire chord, or you can pick at the individual strings within a chord to play individual notes. Plucking the strings within the chords and hopping around between chords is really easy and it sounds great. It's like I'm creating a little background music for an MTV StoryTellers -- my own "banter" music. "A few years back, in a little town not too different from this one... I met a girl" -- That sentence is a whole lot more engaging with a little background strumming accompaniment. If you don't feel like strumming and pecking, you can just select one of the "AutoPlay" options which will play a background riff for you and all you have to do is manage the chord changes. Even my two-year-old daughter can pull off a solo concert with this.
As you lay down your different instrument tracks (or live recordings), you can then edit them in the track view to add loops, effects, or adjust volumes, as well as edit the timings. This is the part where I start to go blurry eyed on the whole audio recording skill set, but it's an incredibly easy interface to use and fun to play with.
My favorite feature, however, is one that is currently undocumented. GarageBand for iPad seems to have some limited MIDI support built-in to it. What this means is that with an iPad, and USB Camera Connection Kit, and a compatible MIDI keyboard, you can capture your keystrokes on a physical music keyboard and GarageBand will record the MIDI track which you can use to edit or play back with different instruments. Since it's an undocumented feature, finding a list of supported devices is tricky, but it worked fine with my Yamaha S08 -- if that helps anyone. It's just one more very cool way to get music into the GarageBand app which you can then continue to edit and tweak to your hearts content.
Once your tracks are in GarageBand, you can then export the song file for more advanced editing on the GarageBand desktop app (and presumably other music editing software), but that was beyond my scope -- for now, I'm just going to continue playing a little background music as I tell my family about my day at work.